Re: [PATCH] mm,thp,shmem: limit shmem THP alloc gfp_mask

[Michal Hocko <mhocko@suse.com>]

On Thu 22-10-20 09:25:21, Rik van Riel wrote:
> On Thu, 2020-10-22 at 10:15 +0200, Michal Hocko wrote:
> > On Wed 21-10-20 23:48:46, Rik van Riel wrote:
> > > The allocation flags of anonymous transparent huge pages can be
> > > controlled
> > > through the files in /sys/kernel/mm/transparent_hugepage/defrag,
> > > which can
> > > help the system from getting bogged down in the page reclaim and
> > > compaction
> > > code when many THPs are getting allocated simultaneously.
> > > 
> > > However, the gfp_mask for shmem THP allocations were not limited by
> > > those
> > > configuration settings, and some workloads ended up with all CPUs
> > > stuck
> > > on the LRU lock in the page reclaim code, trying to allocate dozens
> > > of
> > > THPs simultaneously.
> > > 
> > > This patch applies the same configurated limitation of THPs to
> > > shmem
> > > hugepage allocations, to prevent that from happening.
> > > 
> > > This way a THP defrag setting of "never" or "defer+madvise" will
> > > result
> > > in quick allocation failures without direct reclaim when no 2MB
> > > free
> > > pages are available.
> > 
> > I remmeber I wanted to unify this in the past as well. The patch got
> > reverted in the end. It was a long story and I do not have time to
> > read
> > through lengthy discussions again. I do remember though that there
> > were
> > some objections pointing out that shmem has its own behavior which is
> > controlled by the mount option - at least for the explicitly mounted
> > shmems. I might misremember.
> 
> That is not entirely true, though.
> 
> THP has two main sysfs knobs: "enabled" and "defrag"
> 
> The mount options
> control the shmem equivalent options
> for "enabled", but they do not do anything for the "defrag"
> equivalent options.

Yeah, the situation is quite messy :/

> This patch applies the "defrag" THP options to
> shmem.

I am not really objecting I just do remember some pushback. My previous
attempt was to unify everything inside alloc_pages_vma IIRC.

> > [...]
> > 
> > > diff --git a/mm/shmem.c b/mm/shmem.c
> > > index 537c137698f8..d1290eb508e5 100644
> > > --- a/mm/shmem.c
> > > +++ b/mm/shmem.c
> > > @@ -1545,8 +1545,11 @@ static struct page
> > > *shmem_alloc_hugepage(gfp_t gfp,
> > >  		return NULL;
> > >  
> > >  	shmem_pseudo_vma_init(&pvma, info, hindex);
> > > -	page = alloc_pages_vma(gfp | __GFP_COMP | __GFP_NORETRY |
> > > __GFP_NOWARN,
> > > -			HPAGE_PMD_ORDER, &pvma, 0, numa_node_id(),
> > > true);
> > > +	/* Limit the gfp mask according to THP configuration. */
> > > +	gfp |= __GFP_COMP | __GFP_NORETRY | __GFP_NOWARN;
> > 
> > What is the reason for these when alloc_hugepage_direct_gfpmask
> > provides
> > the full mask?
> 
> The mapping_gfp_mask for the shmem file might have additional
> restrictions above and beyond the gfp mask returned by
> alloc_hugepage_direct_gfpmask, and I am not sure we should just
> ignore the mapping_gfp_mask.

No, we shouldn't. But I do not see why you should be adding the above
set of flags on top.

> That is why this patch takes the union of both gfp masks.
> 
> However, digging into things more, it looks like shmem inodes
> always have the mapping gfp mask set to GFP_HIGHUSER_MOVABLE,
> and it is never changed, so simply using the output from
> alloc_hugepage_direct_gfpmask should be fine.
> 
> I can do the patch either way. Just let me know what you prefer.

I would just and the given gfp with alloc_hugepage_direct_gfpmask

-- 
Michal Hocko
SUSE Labs